The invention relates to an optical apparatus for determining the angle at which light emanates to a surface from a scanning light spot.
It is already known to continuously scan at right angles to the direction of travel by means of a light beam emanating from a laser e.g. webs of fabric or paper which have to be monitored for errors such as surface irregularities. Thus, there is a light spot on the web from which light is reflected in various directions depending on the nature of the surface irregularity. In total, the light emanating from the light spot is reflected in a solid angle whereby the intensity of the light beams in the individual directions is in all characteristic of the surface state. It is already known that the receiving arrangements which monitor the light spot and which generally comprise a light conducting rod (hereinafter sometimes called "light rod") extending parallel to the scanning direction, together with a cylindrical lens positioned in front of the rod, can be arranged at various angles in order to provide the most favourable reception conditions. Generally, the light intensity received by the receiving arrangement is greatest in the direction of the angle of reflection and reduces to a greater or lesser extent at angles diverging therefrom.
The disadvantage of all known receiving arrangements for such scanning light spots is that reception is generally restricted to one or possibly two directions relative to the surface.